It should be treated as `uint32_t` since DOM API does so.  However, there are
some exceptions:

* Result of `nsINode::ComputeIndexOf()`
* Result of `nsAString` methods

They return `-1` as not found, and anyway, they cannot treat large integer
than `INT32_MAX`.  Therefore, this patch does not touch around them.

Differential Revision: https://phabricator.services.mozilla.com/D118933